Magnesium is the eighth most abundant element in the earth's crust although not found in its elemental form. It is a lightweight material of choice because it is one third lighter than aluminum and it has the highest strength-to-weight ratio of any of the commonly used metals. Magnesium is also approximately one-quarter the density of steel and zinc, and has a number of advantageous performance characteristics, including impact resistance and vibration damping capability compared with other competing materials. In addition, die casting of magnesium offers many design and process advantages which augment magnesium's economic attractiveness as a lightweight material.
Magnesium metal is conventionally obtained from electrolysis of anhydrous molten magnesium chloride in a sealed cell. One of the key challenges of producing magnesium metal is not so much the electrolysis process itself than the treatment and conditioning of the starting material containing the magnesium to produce magnesium chloride sufficiently pure to be subsequently electrolyzed.
Conventional methods of generating magnesium chloride solutions or brines include the evaporation of water from salt lake water or seawater. Such process is costly because of the huge amount of water required, magnesium being present therein only in low concentration, typically less than 0.2%, with the exception of the Dead Sea, which contains about 3.5% of magnesium.
Serpentine, commonly known as asbestos tailings, contains a significant amount of magnesium therein, generally from about 20 to 25% by weight. However, these tailings also contain silica and quartz derivatives as major components, and many processes for extracting magnesium from serpentine have been confronted with the generation of silica gel or other siliceous residues that are difficult to filter, thus significantly hindering the recovery of magnesium.
Several processes have been published for magnesium extraction from serpentine or other siliceous bearing materials, but they all require stringent and difficult experimental conditions, and the resulting magnesium chloride solution still contains significant amounts of impurities that must be removed before being considered as a feed material suitable for a magnesium electrolysis cell.
Recently, a new method for the production of a magnesium chloride solution from siliceous materials has been proposed in U.S. Pat. No. 5,091,161. The method involves leaching the siliceous material in a first reactor with a hydrochloric acid solution at a pH lower than 1.5 and a temperature higher than 50.degree. C. to prevent the formation of silica gel (leach step). The leaching can be carried out in a continuous manner when the siliceous material is fed continuously in the reactor. The leaching solution is continuously transferred in a second reactor wherein magnesia is added continuously to raise the pH to 4-7 and precipitate the bulk of the impurities from the solution, still without forming silica gel in the reactive medium (neutralization step), and a solid/liquid separation is performed thereafter. Subsequently, caustic soda is added to increase the pH to 6-7, (purification step), and a further solid/liquid separation is performed. Chlorine gas is sparged through the slurry prior the addition of caustic soda to oxidize any remaining iron to the ferric state and most of the manganese to solid magnesium dioxide. At this stage, minor elements such as manganese, nickel and boron still remain in solution. These impurities must all be removed since they are highly detrimental to the magnesium chloride electrolysis process. Therefore, a last purification step is required to obtain a sufficiently pure magnesium chloride solution. This is achieved by passing the solution in an ion-exchange column to remove these impurities.
Although the process of this patent is a significant step forward over those known previously for extracting magnesium from siliceous materials, there is still room for improvement. For example, ion-exchange columns are extremely costly to install and maintain, because of the necessity to either regenerate the resin regularly, or systematically replace it. Further, such columns are likely to represent a major bottleneck to the overall production of magnesium because of the limited throughput that it can accept.
There is therefore a great need to develop a method for the extraction of magnesium from magnesium-containing materials not suffering from the drawbacks mentioned above.